This invention relates to suppression of noise sound waves emitted from a jet engine, more particularly to the suppression of noise propagation from a gas turbine engine fan assembly downstream through the bypass duct thereof.
Noise has been a significant negative factor associated with the commercial airline industry since the introduction of the aircraft gas turbine engine. Considerable effort has been directed toward quieting aircraft engines.
Gas turbine engine noise is generated by two primary sources. First, there is noise associated with viscous shearing of rapidly moving gases exhausted into the relatively quiescent surrounding atmosphere. In turbo fan aircraft engines, such gases are emitted from the fan and core nozzles at the rear of the engine. Various approaches have been utilized to reduce this xe2x80x9cshearxe2x80x9d noise, most approaches incorporating mixers to co-mingle fan and exhaust gases with each other and with the surrounding environment.
The second source of noise is the rotating turbo machinery itself, as the result of rapidly rotating blade rows disposed within the gas stream. Fans and compressors include at least one row of a plurality of circumferentially spaced apart rotor blades for compressing air channeled therethrough, and a row of circumferentially spaced apart stator vanes axially spaced apart from the rotor blades. The rotor blades rotate about a longitudinal centerline axis of the engine at a rotational speed and effect a tonal noise at a blade passing frequency (BPF). The aerodynamic interaction of rotor blade-wakes and stationary vanes adds a significant contribution to the noise produced by the jet gas turbine engine. Interaction tones are generated in the region between the rotor blades and the stator vanes, within the annular duct surrounding the blades and the vanes, conventionally known as spinning mode tones or noises.
The blade rotation-wakes of the air compressed by the rotor blades form rotational pressure fields and impinge on the stator vanes, thereby creating the spinning mode tones.
The spinning mode tones occur at discrete frequencies including the fundamental blade passing frequency BPF, alternatively referred to herein as the first harmonic, and higher order frequencies including the second, third and higher harmonics. When this spinning mode speed is fast enough to reach a local Mach number to be greater than about 1.1, the spinning mode tones will propagate outside, both upstream through the duct inlet and downstream through the bypass duct, enhancing BPF tone levels generated directly by the rotor blades.
In order to reduce the spinning mode tone noises, it is well known in the art to direct the spinning mode tones to impinge on the walls of the interior of the engine, including the bypass duct, which is lined with a sound absorbent material. This technique causes the spinning mode tones to decay before exiting the engine. Normally, the bypass duct liners are tuned for the BPF tone and the higher harmonics cannot be efficiently suppressed. In addition, because of the limited duct wall area within most conventional jet engines, such acoustical wall treatment has only made small reductions in fan noise levels, and this is compounded by engine nacelle length-to-radius ratios becoming smaller.
Efforts have been made to seek alternative solutions to reduce engine fan noise levels. Reduction of the spinning mode noise can be achieved by reduction of the production processes at the source of the noise which reduces the incident aerodynamic unsteadiness or the mode generation from fan-stator interactions. It is conventionally known in the art to select the number of vanes and the number of blades to create a spinning mode propagation cut-off phenomenon, as described, for example, by Gliebe et al. in their U.S. Pat. No. 5,169,288 issued on Dec. 8, 1992. In practice, for the spinning mode propagation cut-off a number V of vanes and number B of blades are selected to achieve V greater than 2.3B. In some designs, however, particularly in high bypass turbine fan engines requiring a relatively large number of rotor blades, a cut-on fan-stator V less than 2B may be selected in order to find a compromise with other design criteria. In such cases, the spinning mode is always cut-on, resulting in increasing the fan BPF tone noise and its higher harmonics.
In U.S. Pat. No. 4,300,656 issued to Burcham on Nov. 17, 1981, Burcham describes an acoustic noise elimination assembly having the capability of disrupting the continuity of rotating fields of sound pressures forwardly projected from fans or rotors of a type commonly found in the front or compressor first stage of air-breathing engines, when operating at tip speeds in the supersonic range. The assembly includes a tubular cowl defining a duct for delivering an air stream axially into the intake of a jet engine and sound barrier, defined by a plurality of intersecting flat plates or struts having a line of intersection coincident with a longitudinal axis of the tubular cowl, which serves to disrupt the continuity of rotating fields of multiple tonal components of a noise.
Nevertheless, in addition to the conventional bypass duct acoustic liner, few attempts have been made to reduce the rearward propagation of a fan BPF tone and its harmonics which increase a total noise level emitted from the rear of an engine, thereby more severely affecting the environment especially in a take-off condition. Therefore, it is desirable to develop new methods and apparatus to attenuate the fan BPF tone and its harmonics within the bypass duct of gas turbine engines.
It is an object of the present invention to provide a method and an apparatus to suppress the fan BPF tone noise and higher harmonics thereof within bypass ducts of gas turbine engines.
It is another object of the present invention to provide an apparatus to disrupt continuity, destroy a symmetrical pattern and absorb sound energy of a spinning mode of sound pressures in a bypass duct of a gas turbine engine.
It is a further object of the present invention to provide a gas turbine engine fan assembly with a low level fan BPF tone noise and higher harmonics thereof propagating rearwardly out of the engine.
It is a still further object of the present invention to provide a method and an apparatus for suppressing rearward noise propagation of a fan BPF tone and higher harmonics thereof through a bypass duct of a gas turbine engine without substantially affecting the thrusts provided by the air flow when discharged from the bypass duct.
In general terms according to the present invention, rearward noise propagation of fan BPF tone and higher harmonics thereof through an annular bypass duct of a gas turbine engine is suppressed in the annular bypass duct downstream of a plurality of stator vanes by disrupting continuity, destroying a symmetrical pattern and absorbing sound energy of a spinning mode of sound pressures imbedded in an air flow downstream of the stator vanes without substantially affecting a thrust provided by the air flow when discharged from the annular bypass duct.
In accordance with one aspect of the present invention an apparatus for suppressing rearward noise propagation of a fan BPF tone and higher harmonics thereof through an annular bypass duct of a gas turbine engine comprises at least one perforated baffle plate, preferably a plurality of perforated baffle plates, extending generally in an axial direction with respect to the gas turbine engine and adapted to divide a major section of the annular bypass duct downstream of a plurality of stator vanes, into a plurality of axial flow-path segments. The axial flow-path segments are in fluid communication with one another through the perforations in the at least one baffle plate. When the plurality of the perforated baffle plates are provided, the axial flow-path segments are in fluid communication with adjacent ones through the perforations in the respective baffle plates.
It is desirable that the perforated baffle plates are unevenly circumferentially spaced apart from one another so that the axial flow-path segments are formed in an asymmetrical pattern.
In one embodiment of the present invention the perforations in the respective baffle plates have a percentage of opening area (POA) ranging from 20% to 50%, and are in a staggered hole pattern. It is also preferable that the perforations in the respective baffle plates include holes having uniform diameters and a ratio of individual hole diameter to plate thickness between 0.5 and 2.0.
In accordance with another aspect of the present invention, there is a gas turbine engine fan assembly which comprises a plurality of circumferentially spaced rotor blades; a plurality of circumferentially spaced rotor vanes axially spaced apart from the rotor blades; an annular duct surrounding the rotor blades and stator vanes, and having an inlet for receiving an air flow and an outlet for discharging at least a portion of the air flow compressed by the rotor blades and passed over stator vanes; and means installed in the annular duct downstream of the stator vanes for disrupting continuity, destroying a symmetrical pattern and absorbing sound energy of a spinning mode of sound pressures imbedded in the air flow, downstream of the stator vanes, without substantially affecting a thrust provided by the air flow when discharged. The means according to one embodiment of the invention comprises at least one perforated baffle plate extending generally in an axial direction with respect to the fan assembly, dividing a major section of the annular duct downstream of the stator vanes into at least two axial flow-path segments. The two axial flow-path segments are in fluid communication with each other through the perforations in at least one baffle plate.
According to another embodiment of the present invention, the means comprises a plurality of perforated baffle plates extending generally in an axial direction with respect to the fan assembly, each adapted to span across an annulus defined between inner and outer walls of the annular bypass duct. Thus, a major section of the annular duct downstream of the stator vanes is divided into a plurality of axial flow-path segments. The axial flow-path segments are in fluid communication with adjacent ones through the perforations in the respective baffle plates. The perforated baffle plates are preferably separated axially from the stator vanes by a space relatively small with respect to a length of the perforated baffle plates. Each of the perforated baffle plates may be slightly curved so that a downstream section of the plate is gently and gradually deviated from an axial orientation in a circumferential direction opposite to a rotation of the rotor blades. The circumferential deviation is preferably small relative to the length of the perforated baffle plate. It is also preferably to have the perforated baffle plates unevenly circumferentially spaced apart from one another so that the axial flow-path segments are formed in a asymmetrical pattern.
The present invention provides a solution effective for suppressing rearward noise propagation of fan noise in a bypass duct of a gas turbine engine without substantially affecting the thrust provided by the air flow when discharged from the bypass duct. Other advantages and features will be better understood with reference to preferred embodiments to be described hereinafter.